Ok so it turns out that the speedometer gear is worn out and we need a new one. when we could get it to work it was about 20mph too fast going only 55-60mph (it read 75-80). the current gear had 20 teeth and we were wondering about how many teeth the new one should have. We know it needs more teeth but we dont know how many. any suggestions?

its a 1980 Z with a TH350

 

Your local Chevy dealer parts guy should be able to help with this, at least they used to. The number of teeth on the gear is in relation to the size of your differential. Have that information with you when you talk to him. I think you already know how to tell the size of the gear, its either on a tab on the rear end, or you can jack the car up, use a piece of chalk and mark the pinion yoke or the driveshaft. turn the rear tire one full turn while watching the mark and see how many times it turns during that one revolution of the tire, 3 3/4 turns is a 3.73, a little more than 4 turns is probably a 4.10-4.11. Hope this helps you out. Bob